The present invention has been developed for its particular applicability to the semi-conductor and micro-electronics industries, and in particular to the cleaning of contaminated substrates, including, for example, semi-conductor wafers of silicon and of gallium arsenide, multiple chip carriers, flat panel displays, magnetic hard disks, MEMs and other electronic devices. Many methods have been developed to clean such surfaces. Techniques include the use of solvents or chemical cleaning for removing contaminant films from surfaces, the use of high energy sonic waves, and combinations thereof Solvents for chemicals may be applied as gas jets or liquid spray.
More recently, cryogenic aerosols have been developed for jet spraying against surfaces, particularly within the semi-conductor wafer industry for particulate decontamination. Cryogens that have been used for removing the particulate contamination include argon, carbon dioxide and water. The idea behind cryogenic aerosols is to provide a jet of frozen crystals traveling at subsonic or supersonic speeds. The formation and size of the crystals depends on the thermal dynamic conditions including the pressure, temperature, flow and the crystal forming technique which depends largely on the initial phase of the supplied substance and the nozzle design. Carbon dioxide and water have been used in certain applications; however, silicon wafer cleaning requires high purity and the ability not to damage the surface of the silicon wafer. Thus, argon aerosol now seems particularly useful for semi-conductor wafer cleaning. For example, U.S. Pat. Nos. 5,377,911 to Bauer et al, 5,062,898, 5,209,028, 5,294,261, to McDermott et al disclose the use of cryogenic aerosols which may include argon combined with nitrogen. U.S. Pat. Nos. 4,747,421 to Hayashi and 4,806,171 to Whitlock et al describe apparatuses for cleaning substrates using carbon dioxide aerosol crystals.
A simple schematic of the cryogenic aerosol cleaning system is illustrated in FIG. 1, including a silicon wafer surface 1 and a jet impingement nozzle 2. Nozzle 2 includes a plurality of orifices along its length from which the aerosol jet spray is propelled toward the silicon wafer surface 1. Typically, the orifices are configured so that the aerosol jet stream impinges the silicon wafer 1 at a predetermined angle. The aerosol contains aerosol crystals 3 which are suspended within the aerosol gas illustrated by the lines extending from the nozzle to cross the silicon wafer surface 1 and from the silicon wafer surface 1 at the arrowheads. Contamination particles 4 are illustrated on the silicon wafer surface 1 and are also shown being carried away from the aerosol gas jet stream from the silicon wafer surface 1. Typically, the jet impingement nozzle 2 is fixed in position and at a specific angle so that the aerosol jet stream including the aerosol crystals 3 impinge the silicon wafer surface 1 which is supported to be movable relative to the fixed position of the jet impingement nozzle 2. Usually, the silicon wafer having the surface 1 is mounted in a manner so that it can translate below the jet impingement nozzle 2 so that the entire silicon wafer surface 1 can be cleaned. Aerosol crystals 3 carried in the aerosol gas impinge the surface of the silicon wafer 1, cause the removal of contaminate particles 4, and the jet stream carries the contaminate particles 4 away from the silicon wafer surface 1. As described above, the aerosol may comprise cryogenic aerosol clusters as the aerosol crystals 3 or any of the other particles or liquids known for cleaning.
In a cryogenic cleaning apparatus, the nozzle 2 and silicon wafer, which would be supported by a movable chuck, are provided within an aerosol cleaning chamber. The aerosol cleaning chamber is provided in vacuum during the cleaning process in a manner to control formation of cryogenic aerosol crystals 3. More specifically, the inert substance, such as an argon and nitrogen mixture, is supplied to the nozzle 2 and is expelled from the jet impingement nozzle 2 into the vacuum cleaning chamber within which the cryogenic aerosol crystals 3 and aerosol gas jet stream are formed.
The cryogenic aerosol crystals 3 are primarily formed by evaporative cooling. Evaporative cooling relies on small liquid droplets that freeze prior to impinging the silicon wafer surface 1. The small liquid droplets are formed from larger droplets that are atomized by the high pressure gas that expands from the nozzle orifices. The small liquid droplets (the aerosol spray) freeze into crystals due to the pressure drop between the nozzle and the silicon wafer. Crystals formed by evaporative cooling are generally of about one to ten microns (1-10.mu.) or larger in diameter. Less significantly, cryogenic aerosol crystals are also formed via Joules Thompson cooling, which is homogeneous nucleation of crystals based upon the temperature drop associated with the expansion within the aerosol cleaning chamber, as described in the above-noted Bauer U.S. Pat. No. 5,377,911. This nucleation provides much smaller and less effective crystals generally in the order of 0.01.mu. in diameter.
Thus, in order to achieve the primary formation of the crystals by the atomization of liquid droplets into small liquid droplets and the subsequent freezing, the nozzle design must distribute the liquid uniformly along the length of the nozzle. Uniform distribution ensures the formation of the larger droplets along the length of the nozzle to be atomized, frozen, and carried by the aerosol gas.
As shown in FIGS. 2 and 3, a jet impingement nozzle 2 is shown at a fixed angular orientation. The inert substances provided within the interior of the nozzle 2, and the liquid forms a pool therein substantially to the level of the line of orifices. The aerosol spray is expelled from the orifices as noted above. It is important to provide a uniform spray along the longitudinal length of the nozzle for uniform processing. As shown in FIG. 3, the liquid pool extends along the portion of the longitudinal length that is illustrated. It is believed that waves move through the liquid pool, as also indicated in FIG. 3. These waves are believed to cause what has sometimes been observed as a wavering effect of the intensity of the aerosol spray along the length of the nozzle. This effect is referred to as a "walking" effect. Although the instantaneous uniformity of the nozzle is poor when walking is observed, this phenomenon does not significantly effect the uniformity of the processing since the waves move back and forth along the longitudinal length of the nozzle.
A consequence, however, of providing a fixed nozzle is the creation of a specific impingement angle. When cleaning or otherwise treating a substrate surface having surface features, patterns or vias, the aerosol stream at the set impingement angle may not adequately clean surfaces of the features. For example, the cleaning of contaminants from deep trenches and other surface features may be more thoroughly accomplished by orienting the aerosol spray direction nearly perpendicular to the substrate surface, while cleaning debris from a flat surface might require orientation of the aerosol spray at a very shallow grazing angle to the substrate surface. Furthermore, even with flat surfaces, different angles of impingement may be more effective because of the shape of the contaminant particles and the way that they are adhered to the flat surfaces.
In FIG. 4, a similar nozzle as that described above is illustrated but which is fixed in an orientation to provide a substantially perpendicular aerosol spray directed at a substrate surface. Specifically, the aerosol spray direction is illustrated in the direction of gravity. In this situation, the liquid pooling effect, discussed above, and thus the formation of cryogenic aerosol crystals is compromised. The liquid pooling is non-uniform and may even decrease to nothing, as illustrated on the right side, regardless of whether the cryogenic fluid is delivered at one or both ends of the nozzle or at any point along its length. With non-uniform liquid pooling, formation of cryogenic aerosol crystals at certain locations along the nozzle length can be inhibited and non-uniform processing can occur.
The aforementioned Bauer et al U.S. Pat. No. 5,377,911 discloses a fixed nozzle utilizing a dual chamber. As shown in FIG. 7A of the Bauer et al patent, an upper chamber connects with a lower chamber through a series of orifices, and another series of orifices are provided from the lower chamber from which the aerosol is expelled. However, the purpose of providing the upper and lower manifolds is to eliminate the generation of low pressure points and even distribution of gas passed from the upper distribution manifold to the lower distribution manifold.